Electronic microscope for top illumination of surfaces



March 15, 1960 G. E. BARTZ EI' 7 2,928,943

ELECTRONIC MICROSCOPE FOR TOP ILLUMINATION 0F. SURFACES Filed Sept. 5.1958 4 Sheets-Sheet 1 INVENTORS GU/VTER BQW/N BARTZ WAUER BILL BMWATTORNEYS March 15, 1960 G. E. BARTZ EI'AL 2,928,943

ELECTRONIC MICROSCOPE FOR TOP ILLUMINATION 0F SURFACES Filed Sept. 5.1958 4 Sheets-Sheet 2 INVENTORS GUNTER ERMA! BARTZ WALTER 8/LL wmm mz,

ATTORNEYS G. E. BARTZ ELECTRONIC MICROSCOPE FOR TOP ILLUMINATION 0FSURFACES 4 Sheets-Sheet 3 March 15, 1960 Y Filed Sept. 5, 1958 v Fig.'lb

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\fa M GUNIERERW/NBARTZ WALTERB/LL v 151 mm Arrows IN VENTORS March 15,1960 G. E. BARTZ EIAL 2,928,943

ELECTRONIC MICROSCOPE FOR TOP ILLUMINATION OF SURFACES Filed Sept. 5.1958 4 Sheets-Sheet 4 4a 5a 6a 5/ 7a INVENTORS GUNTER ERM/V BARTZ AWALTER ,B/LL

BYW

ATTORNEYS I ELECTRONIC ,MICROSCOPE FOR TOP ILLUMI- NATION F SURFACESGiinter Erwin Bartz, Dutenhofen, and Walter-Bill, Naunhelm, Germany,assignors to Ernst Leitz, G.m.b.H., WetzlanGermany Y ApplicationSeptember5i1958, Serial No. 759,162

priority, application Germany September 11, 1957 8 Claims. (Cl. 250-495) This invention relates to an electronic microscope suitable formaking surfacesof objects visible by means of top illumination. a

It is already known in the art to make an object surthe surface in theform of material layers which lead to a filling in of depressions inthat surface equal to a snowing-in effect, thus progressively hidingdetails of the original surface in the resulting image.

It is the object of our invention to avoid these and other drawbacks ofthe presently known apparatus, and to provide an electronic microscopefor top illumination of surfaces, which is simpler both in constructionand operation than the existing devices while being free in particular,from the snowing-in effect of ion beam microscopes, and subjecting thesurface of the object only to a moderate load.

This object is achieved by the electronic microscope according to theinvention which is adapted for forming direct images of. the surface ofan object by means of secondary electrons, and provided with aconventional electronic beam-operating system comprising an acceleratinganode, which system is so disposed as to cause an electron beam to fallunder an oblique angle on to the surface area of the object to bedepicted, and which microscope is further provided with a shielding cageor can adapted for excluding electrostatic fields in its interior andhaving a plurality ofopenings and being rotationsymmetrical in shape andenclosing the object, While being at the same time centered about theoptical axis of the secondary emission path serving to produce theimage, and being at the same electrical potential as the object itself.

According to another feature, the objective system of the electronicmicroscope according to the invention is formed by the aforesaidshielding can and the object together with a ground-connected electrodedisposed behind the can in the electronic path from the object to theimage observation means, which latter is preferably a conventionalfluorescent screen,

In other embodiments of the electronic microscope according to theinvention, the aforesaid electrode forming part of the objective systemis replaced by other lens means in the imaging path from the object tothe, image-producing screen means, which other means con sist either ofan electrostatic unipotential lens, or an electrostatic immersion lens,or a magnetic lens or lens system forming part of the objective system.

Furthermore, the electron microscope according to the invention maycomprise an additional electron beami' d States P t ,Q i

:01 can permits a straight-lined travel of the illuminating a Wehneltcylinder andan accelerating anode.

7 2,928,943 Patented Mar. 1 5,

ing source, which ion source is almost, but not quite at the samepotential as the object, and is so disposed in the'device according tothe invention that the direction of the ion beam approximately coincideswith the direction of the primary electron beam directed towards theobject. The emitter system for generating the primary electron beamcomprises, for example, an electron source, Such a system is, forinstance, described in Manfred von Ardenne: Tabellen derElektronenphysik, VEB Deutscher "Verlag der Wissenschaften, Berlin(1956), vol. I, p. 131.

It is advisable to have the electron emitter system directed at anoblique angle'toward the object.

The body of the shield can or cage enclosing the object and protectingthe same against undesirable electrostatic fields is substantiallysymmetrical with regard to an axis of rotation which, preferably, isidentical with the optical axis of the image-transmiting' system formedby the paths of the secondary electronic beams, and is provided with aplurality of. openings. Primary electrons are permitted to enter theshielding can by a first, lateral opening, while secondary electronsleave the can by way of another opening provided close to the object inthe A third opening or bore in the cage Wall serves for introducing andwithdrawing the object from the can and for unimpeded displacement ofthe object carrier or holder,

In a similar manner, several independent illuminating systems fordirecting primary electron beams toward dilferent portions of the objectsurface may be provided in the device according to the invention and, inthis case, a correspondingly larger number of openings will have to beprovided in the can wall, so that the object may be top-illuminatedseverally either at the same time, or in successive order. I

The field-free space in the interior of the shield cage beam or beams.Furthermore, the can provides a reliable safeguard againstdischarges,for the high field intensity at the object which latter usually bearscorners and/ or edges, is only utilized at the spot or area underobservation,- and the danger of an electrical arc-over is substantiallyeliminated or at least greatly reduced. It is correspondingly possibleto operate the microscope with extremely high field intensities, forinstance in the order of 50 kilovolts/cm., which, of course, signifies aconsiderable increase in the resolving power of the emission microscope,since the latter increases in linear function with the field intensityat the place under observation.

The ejection of image-forming secondary electrons from the object byprimary electrons instead of the conventionally used ions olfers severaladvantages. First of all, the object rating is considerably less whenirradiating the object with electrons instead of ions. Secondly, as hasbeen mentioned above, the snowing-in effect is avoided.

Other advantages reside in the fact that a single high voltage sourcewill sufiice for providing voltage to the system, and that the angle ofthe incident primary beam relative to the object can easily be alteredby changing the object voltage. Other advantages are of avacuum-technical nature, it being possible to obtain an extremely purevacuum in the interior of the electronic tube which could not beattained if an ion beam is directed into the interior of the apparatus.

On the other hand, when objects of electrically'nonconductive orsemiconductive materials are to be made visible, it may be desirable toprevent the object from .tron beam-producing system. This ion-beamproducing source should be provided with means for adjusting theintensity and direction of the beam, the latter to co1nc1deapproximately with that of the primary electron beam,

and should be held approximately at object potential, as statedhereinbefore.

spasms The invention will be still further explained with the aid of theaccompanying drawings in which Figure 1 is a schematical sectional viewof an embodiment of the top-illumination electronic microscope accordingto our invention;

Figure 1a is a longitudinal sectional view of the micro- I :metric meansfor adjusting the diaphragm 27 suitable for adjusting the object-holder9;

Figure 1b is a cross sectional view of the micrometric adjusting meanstaken along the lines IIII in Figure la;

Figure 2 is a partial sectional view of a somewhat different embodimentof certain electrode means used in the electronic microscope shown inFigure 1;

Figure 3 shows in partial sectional view another embodiment of theelectrode means in the electronic microscope shown in Figure 1; and

Figure 4 illustrates a third embodiment of the electrode means in apartial sectional view of the electronic microscope according to theinvention.

Figure 5 is a partial, schematical sectional view of amodification ofthe primary electron beam generating system in the electronic microscopeof the present invention;

Figure 6 is a partial, schematical sectional view of yet anothermodification of the primary electron beam generating system in theelectronic microscope of the present invention;

Figure 7 is a schematical, sectional view of another embodiment of theinvention, similar to Figure 1, but

f showing another preferred embodiment of the top-illuminated electronicmicroscope of the present invention having a plurality of electron beamgenerating systems and being equipped with an ion beamgenerating-system.

Figure 7a is a schematical, cross sectional view, taken along II ofFigure 1.

More particularly, the embodiment of the electron microscope accordingto the invention as illustrated in Figure 1 comprises a microscope hullor casing 1 having a main tube 2 which is cocentrically arranged aboutthe optical axis of the object-image path, and a lateral angular I tube3 which houses the primary electron beam generating system consisting ofan electron emitting cathode 4, a modulating (Wehnelt cylinder)electrode 5 and an accelerating anode 6. In the path of the primaryelectron stance in the form of an annular body of high tensioninsulating material, in which there is mounted a shield .can-12.

, Can 12 is symmetrical to its axis of rotation which coincides withoptical axis 10, and is preferably made of stainless steel, and freefrom corners or edges and of a highly polished, completely smoothsurface, whereby the formation of corona discharges is substantiallysuppressed.

beam toward the approximate center of the casing 1, 3

Can 12 is electrically connected, on the one hand, by means of a numberof sliding contacts 13 (only one of which is shown in Figure 1) withobject 8, and, on the other hand with a source of high voltage potentialat 14.

Openings 15, 16 and 17 are provided in the wall of can 12, of whichopening 15 is provided in a neck portion 18 of can 12, the front wall ofwhich is somewhat flattened at 19 in a plane which is vertical to theoptical axis 20 of the illuminating system (see Figure 7).

Opening 16 is provided in an inwardly curved or conical wall' portion 21of can 12 whereby the opening is located close to the surface of object8 where the primary electron beam along axis 20 impinges on the latter.

This opening 16 is centered with extreme accuracy on the common opticalaxis through the subsequently described elements of the imaging system.

The further, larger opening 17 in the wall of can 12 opposite. opening16 serves for the introduction, withdrawal and unimpeded displacement ofobject holder 9 in any lateral direction, and for the evacuation of theinterior of can 12, which is efiected together with the interior ofeasing 1 by means of a vacuum pump (not shown) connected to outlet pipe22.

At its free end main tube 2 bears a fluorescent screen 23 or similarmeans for making the electron-beam produced image of the top-illuminatedobject surface area visible.

An electrode 24 which, is connected to ground via line 25, is located inthe path of the secondary electron beams justing means which permit thedistance between the opening 16 of can 12 and the electrode to bealtered. A micrometrically adjustable or iris diaphragm 27 is disposedat the focal point of electrode 24 behind the latter as seen from theobject 8. Between the diaphragm 27 and fluorescent screen 23, there isdisposed an electronic lens 28 of known type which serves as aprojective.

Such lenses are described, for instance, in Von Ardenne, op. cit., vol.I, p. 407, 412, 416.

The adjustment of diaphragm 27 can be effected with cross slidemacrometric adjusting means known per se and shown, for example, inFigures 1a and 1b. The diaphragm 27 can be displace in the directions ofarrow 63 by means of the adjusting screw acting against a spring 61 andit can be moved in the directions of arrow 62 by means of the screw 64engaging the ratchet portion 65 provided in the slidable sleeve 66within the 50 cylinder 2.

The analogous micrometric adjusting means can be used for displacing theobject-holder 9.

The electronic microscope shown in Figure 1 operates as follows:

Primary electrons are emitted from the system of cathode 4, modulated byelectrode 5 and accelerated by anode 6, and then focussed by condenser 7in the direction toward object 8 along optical axis 20.

A voltage, the so-called object voltage is applied at both the object 8and shield can 12, which is positive with regard to the potential ofcathode 4 and can be taken from potentiometer 29 and varied, forinstance,

between 0 and 5000 volts.

Since the can is connected with potentiometer 29 having a voltage in theorder of 2 kilovolts, whereas the anode 6 has a voltage in the order of50 kilovolts (see Figure 1), the electrons of the primary beam have totravel against a negative voltage and are thus braked upon approachingopening 15 of can 12 down to a speed which corresponds to the positivevalue of the object voltage or landing voltage of the primary electronson the object surface spot or area to be topilluminated. In the spacefree from electrostatic field which exists in the interior of can 12,the primary electron beam travels in a straight line toward, the

. aforesaid object spot b1 area. The electrons which impinge on thisspot or area, eject froin the same secondar electrons which travelsubstantially along opti- 'e'al'axis 10. 7

Together with can 12 being at even potential with object 8, the normallygrounded electrode 24 produces an. axially symmetrical acceleratingfield, which grips "through opening '16 of can 12 and accelerates thesecondary electrons leaving the surface of the object from a speed ofabout 2 kilovolts up to 50 kilovolts. This acceleration is necessary inorder to create an image and to obtain the requisite speed in order tocreate an image on screen 23.

This accelerating field also causes a deflection of the illuminatingprimary electron beam in the vicinity of opening. 16 in such a manner,that the angle of incidence a of the primary beam with the objectsurface becomes smaller. p

H A smaller angle of incidence is desirable whenever the surface of theobject is comparatively planar, it is undesirable whenever the surfaceof the object is irregular and has indents and projections. In that caseundesirably elongated shadows would be produced which jcjan be avoidedby choosing a greater angle of incidence. This can be done by changingthe voltage of the voltage sources by means of the potentiometer 29.The. angle of incidence is much smaller by lowering the voltage at thepotentiometer '29 and vice versa.

-In case of substantially planar objects an object voltage in the rangeof 1000-2000 volts is chosen; in case of objects with more irregularsurfaces the object voltage is to be inthe order of 4000-5000 volts. Inboth instances the accelerating voltage is in the order of from 40-50kilovolts. The angle of incidence a is in the order of 20.

By selectinga suitable landing voltage and correspondinglyadjusting theemission of primary electrons fromthe electron beam generating system 4,5, 6, it is, however, possible to provide for optimal conditions ofillumination.

Can "12, electrode 24 and the object 8 form together an electrostaticfocussing lens which serves as an objective system and produces a realintermediary image. In the focus of the objective behind the latter,seen from the object, the iris diaphragm 27 is preferably provided forreducing aberrations occurring in the image. This "diaphragm may have aninner diameter of about 5 to 20 microns. The real intermediary imageproduced by the objective is enlarged with the aid of the abovementionedprojective lens 28 and appears on the fluores- 5 ing electrode 24-isreplaced by an electrostatic unipotential lens 30, in Figure 3 by anelectrostatic immersion double aperture lens 31 and in Figure 4 by aniron clad magnetic lens 32. In Figure 2, the electrode 33 of uni-'potential lens 30 is connected to ground via line 34, while theelectrode 35 is connected via line 36 to a potentiorneter 37, wherebythe potential at electrode 35* can be varied by means of thepotentiometer 37.

In a similar manner, the electrode 38 of the double aperture lens 31 isconnected via line 39 to ground, while the electrode 40 is connected vialine 41 to potentiom- "titer 42.

The arrangement and operation of unipotential lenses in electronicmicroscopes is described in detail, for instance, in Von Ardenne, op.cit., vol. II, p. 802, double aperture lenses in H. Johannson, inAnnalen der Physik (1933), vol. 18, p. 385 and magnetic lenses in VonArdenne op. cit., vol. II, p. 803-805.

The electronic microscope of the present invention can also be equippedwith a modified primary electron beam generating system as shown, forexample, in Figures 5 and '6.

"Ksshown in Figure 5,-the'-anode 6 mayhavea tubular .75

. 6 elongation 62: extending into the opening 150i cage 12. Thecondenser lens 7 is disposed around the tubular elongated portion 6asubstantially in the middle thereof with an interposed insulating layer7a.

Due to this arrangemennan even narrower pencil of primary electrons iscreated since exterior field influences are excluded. Consequently, thespeed of the primary electrons is more even and hence the speed of thesecondary electrons is also more even thereby creating a clearer image.

It is also possible-to dispense with the condenser lens 7 by disposingthe primary electron beam generating system 4, 5, and 6 close to theopening 15 of cage 12 as shown, for example, in Figure 6. In that casethe cage 12, the accelerating anode 6 and the Wehnelt cylinder 5 form anele'ctro-o'ptical system. The adjustment of cathode 4 can be effectedwith adjusting means of the type shown in Figures 1a and lb.

According to another embodiment the electronic microscope of the presentinvention is equipped with a plurality of primary electron beamgenerating systems as, for example, three systems 50, 51, and 52 asshown in Figure 7. The casing 1 bears three cylinder portions 3,

3a, 3b, housing a cathode 4, 4a, 4b, and a Wehnelt cylinder '5, 5a, 5b,an anode 6, 6a, 6b, and a condenser 7, 7a, 7b, respectively. The opticalaxis is designated with 20, 20a, 20b, respectively. These axes passthrough openings 15, 15a, and 15b defined by the flattened frontportions 19, 19a, and 19b and the surfaces 53, 53a and 53b. The objectholder 9 is disposed within the cage 12.

According to still another preferred embodiment the electronicmicroscope of the present invention is provided with an ion beamgenerating system 55. This system is housed in a further cylinderportion 30 of the casing 1 situated as close as possible to one of thethree cylinder portions 50, '51, or 52. This ion source which is, ofcourse, known per se, consists of a gas inlet pipe 56 with a hollowspace inside. At the end opposite to the entrance 57 of the inlet pipe.there is provided a small exit opening 58. The gas discharge pipe isembedded in an insulating layer 70 within the electrode 59. The gasinlet pipe is connected with the positive pole of a 'voltage source 72of approximately 2000 volts, whereas tive pole of a-further voltagesource 73 of approximately 100 volts. The negative pole of this secondvoltage source is also connected with the cage 12.

This ion generating source operates as follows:

The gas leaving the small opening 58 of the gas inlet pipe 56.results ina gas discharge due to the voltage of about 2000 volts between the inletpipe 53 and the electrode 59 which causes the gas to be ionized. Theions leavingthe opening 75 of the electrode 59 are accelerated towardsthe object holder by the voltage in the range of 10m volts between theelectrode 59 and the cage 12. If a low accelerating voltage is chosen,for example 10 volts, the positive ions will cause a discharging of theobject which is negatively charged by the beam of primary electrons, ifthe object'consists of a non-conductor.

By choosing a high accelerating voltage, for example 100 volts, the ionswill have an etching effect on the surface of the object, for example ametallic object. In this case the beam of electrons may be directed tothe object only for a few seconds since otherwise too many ions wouldcover the object thereby producing the undesirable snow effect. Theetching performed in such a manner ofiers great advantages over theetching done by means of acids since a much finer etching eifect isobtained.

' It will be understood that this invention is susceptible tomodification in order to adapt it to different usages and-conditions,and, .accordingly, -it is desired to comprehend such modificationswithin this invention as may surface of an object by means of secondaryelectrons ejected from said surface toward a fluorescent screen adaptedfor making the imaged object surface visible, and wherein the object andfluorescent screen are disposed along an optical axis of the microscope,comprising means for producing primary electrons, anode means foraccelerating said primary electrons, means for directing said primaryelectrons toward the object, and a shielding can enclosing said objectand provided with openings for the entry of said primary electrons, forthe passage of said secondary electrons from said object toward saidfluorescent screen, as well as for the introduction, withdrawal anddisplacement of the object relative to said can, said can presentingrotation symmetry and being centered upon the aforesaid optical axis,and means for applying the same electrostatic potential to said objectand said can.

2. An electronic miscroscope for direct imaging of the surface of anobject by means of secondary electrons ejected from said surface towarda fluorescent screen adapted for making the imaged object surfacevisible, and wherein the object and fluorescent screen are disposedalong an optical axis of the microscope, comprising means for producingprimary electrons, anode means for accelerating said primary electrons,means for directing said primary electrons at an oblique angle towardthe object and a shielding can enclosing said object and provided withopenings for the entry of said primary electrons, for the passage ofsaid secondary electrons from said object toward said fluorescentscreen, as well as for the introduction, withdrawal and displacement ofthe object relative to said can, said can presenting rotation symmetryand being centered upon the aforesaid optical axis, and means forapplying the same electrostatic potential to said object and said can.

3. An electronic miscroscope for direct imaging of the surface of anobject by means of secondary electrons ejected from said surface towarda fluorescent screen adapted for making the imaged object surfacevisible, and wherein the object and fluorescent screen are disposedalong an optical axis of the microscope, comprising means for producingprimary electrons, anode means for accelerating said primary electrons,means for directing said primary electrons toward the object, ashielding can enclosing said object and provided with openings for theentry of said primary electrons, for the passage of said secondaryelectrons from said object toward said fluorescent screen, as well asfor the introduction, withdrawal and displacement of the object relativeto said can, said can presenting rotation symmetry and being centeredupon the aforesaid optical axis, and means for applying the sameelectrostatic potential to said object and said can; and electrode meansintermediate said can and said fluorescent screen, said electrode meansbeing grounded and constituting together with said can the objectivesystem of the microscope.

4. An electronic microscope for direct imaging of the surface of anobject by means of secondary electrons ejected from said surface towarda fluorescent screen adapted for making the imaged object surfacevisible, and wherein the object and fluorescent screen are disposedalong an optical axis of the microscope, comprising means for producingprimary electrons, anode means for accelerating said primary electrons,means for directing said primaryelectrons toward the object, a shieldingcan enclosing said object and provided with openings for the entry ofsaid primary electrons, for the passage of said secondary electrons fromsaid object toward said fluorescent screen, as well as for theintroduction, withdrawal and displacement of the object relative to saidcan, said can presenting rotation symmetry and being centered upon theaforesaid optical axis, and means for applying the same electrostaticpotential to said object and said vcan; and electrostatic unipotentiallens means intermediate said can and said fluorescent screen, saidelectrostatic unipotential lens means being grounded and constitutingtogether with said can the objective system of the microscope.

5. An electronic microscope for direct imaging of the surface of anobject by means of secondary electrons ejected from said surface towarda fluorescent screen adapted for making the imaged object surfacevisible, and wherein the object and fluorescent screen are disposedalong an optical axis of the microscope, comprising means for producingprimary electrons, anode means for accelerating said primary electrons,means for directing said primary electrons toward the object, ashielding can enclosing said object and provided with openings for theentry of said primary electrons, for the passage of said secondaryelectrons from said object toward said fluorescent screen, as well asfor the introduction, withdrawal and displacement of the object relativeto said can, said can presenting rotation symmetry and being centeredupon the aforesaid optical axis, and means for applying the sameelectrostatic potential to said object and said can; and electrostaticimmersion lens means intermediate said can and said fluorescent screen,said electrostatic immersion lens means being grounded and constitutingtogether with said can the objective system of the microscope.

6. An electronic microscope for direct imaging of the surface of anobject by means of secondary electrons ejected from said surface towarda fluorescent screen adapted for making the imaged object surfacevisible, and wherein the object and fluorescent screen are disposedalong an optical axis of the microscope, comprising means for producingprimary electrons, anode means for accelerating said primary electrons,means for directing said primary electrons toward the object, ashielding can enclosing said object and provided with openings for theentry of said primary electrons, for the passage of said secondaryelectrons from said object toward said fluorescent screen, as well asfor the introduction, withdrawal and displacement of the object relativeto said can, said can presenting rotation symmetry and being centeredupon the aforesaid optical axis, and means for applying the sameelectrostatic potential to said object and said can; and magnetic lensmeans intermediate said can and said fluorescent screen, said magneticlens means being grounded and constituting together with said can theobjective system of the microscope.

7. An electronic microscope for direct imaging of the surface of anobject by means of secondary electrons ejected from said surface towarda fluorescent screen adapted for making the imaged object surfacevisible, and

wherein the object and fluorescent screen are disposed along an opticalaxis of the microscope, comprising a plurality of systems for producingbeams of primary electrons, anode means for accelerating said primaryelectrons, means for directing said primary electrons toward the object,and a shielding can enclosing said object and provided with openings oneeach for the entry of one of said beams of primary electrons, for thepassage of said secondary electrons from said object toward saidfluorescent screen, as well as for the introduction, withdrawal anddisplacement of the object relative to said can, said can presentingrotation symmetry and being centered upon the aforesaid optical axis,and means for applying the same electrostatic potential to said objectand said can.

8. An electronic microscope for direct imaging of the surface of anobject by means of secondary electrons ejected from said surface towarda fluorescent screen adapted for making the imaged object surfacevisible, and wherein the object and fluorescent screen are disposedalong an optical axis of the microscope, comprising means for producingprimary electrons, anode means for acpglerating said primary electronstoward the object,

said object and said 'can, and anqadditional source of slow ions, meansfor applyingto said ion source an e1ectrostatic potential close to thatof said object, said ion source being so arranged in the microscope thatthe direction of the ions produced thereby substantially coincides withthe direction of the aforesaid primary electrons, and comprising meansfor controllingthe intensity, speed and direction of the ion emission.

References Sited in the file of this patent UNITED STATES PATENTSDornfeld Feb. '28, 1950 Reisner June 6, 1950 Weissenberg July 16, 1957FOREIGN PATENTS Germany July 4, 1955

